1. Field of the Invention
The present invention relates generally to automotive internal combustion engine and more specifically to an induction system for an internal combustion engine wherein each combustion chamber is communicated with the induction system through multiple inlet valves.
2. Description of the Prior Art
Japanese Patent Application First Provisional Publication Sho 47-31724 discloses an induction arrangement wherein each combustion chamber of the engine is provided with first and second inlet valves. The first of the valves is arranged to open well ahead of TDC of the exhaust stroke so as to provide a relatively large valve overlap with respect to the timing of a single exhaust valve. The cylinder head in which the valves are disposed is formed with a first inlet port which leads to the first valve. This inlet or induction port is provided with a butterfly type valve which is arranged to cut off communication between the induction manifold attached to the cylinder head and the first inlet valve when the engine is operating at low engine speeds. The cylinder head is further formed with a second inlet port which leads from the induction manifold to the second inlet valve. The second inlet valve is arranged to lift at a timing close to TDC of the exhaust stroke. The second inlet port is not provided with any flow control valves and thus provides constant communication between the induction manifold and the first inlet valve.
As will be appreciated the lift timing of the first valve is arranged to provide good respiration characteristics when the engine is operating at high engine speeds while the second appropriate characteristics when the engine is operating under low load/engine speed conditions such as idling.
However, this arrangement has suffered from the drawback that as the first inlet port is used under both middle and high engine speed operation and the second under low, middle and high engine speeds, a compromise must be struck and the length of both ports selected to provide optimum charging efficiency during middle engine speed operational modes so as to avoid any "flat spots" in engine output as the engine speed rises from low to medium engine speed. This of course reduces the effect of the system at low and high speed engine operation modes.
Japanese Patent Application First Provisional Publication Sho 57-110765 discloses an arrangement similar to that mentioned above but wherein the length and cross section of the induction passages which lead to the inlet valves are selected to provide a ramming effect which improves charging efficiency. However, like the above mentioned arrangement, in order to avoid "flat spots", the sizing of the ports is such as to be a compromise between high and low speed operation so as to obviate a loss of charging efficiency when the engine moves from low to middle engine speed operation. Accordingly, the ramming effect provided by this induction system at low engine speeds is insufficient to generate the desired degree of swirl within the combustion chamber under such conditions. This of course leads to deterioration of engine operation stability especially during idling and the like due to poor flame front propergation and thus enhances the undesirable emission of pollutants such as HC and CO.